A rough terrain vehicle or an all-terrain vehicle which is also called a buggy is a four-wheel off-road vehicle for one person and is used as a recreational vehicle for hunting and trail touring and is also used as a practical vehicle for agriculture. In such an all-terrain vehicle, an engine is mounted in a lateral direction in a vehicle body such that a crankshaft extends in a direction of vehicle width and a power transmission system for transmitting engine power to driving wheels is provided with a forward/backward switching mechanism, a belt type continuously variable transmission, a centrifugal clutch and the like (see Japanese Patent Application Laid-open Nos. 10-297294 and 2002-68070)
In a conventional all-terrain vehicle, an engine is mounted in a lateral direction in a vehicle body such that a crankshaft extends in a direction of vehicle width and on the front side of a rider's seat. Then, the crankshaft, a primary shaft of a continuously variable transmission to which the rotation of the crankshaft is transmitted, and a centrifugal clutch for transmitting the rotation of the crankshaft to the primary shaft at a time when the number of revolutions of the crankshaft is more than a predetermined number are concentrically mounted in the vehicle body. When the crankshaft, the primary shaft and the centrifugal clutch are concentrically arranged in this manner, a shaft length including the crankshaft and the primary shaft arranged concentrically with the crankshaft via the centrifugal clutch is elongated and hence the power transmission system for transmitting the engine power to driving wheels is elongated in size in a direction of width of the vehicle.
In the all-terrain vehicle, the power transmission system is arranged in the region from the front side of a seat toward a portion directly below the seat and a rider sits astride the seat to drive the vehicle. For this reason, when the power transmission system is elongated in size in a direction of width of the vehicle, the rider needs not only to drive the vehicle with his crotch widely opened when he drives the vehicle but also to open his crotch widely when he rides on and off the vehicle. Hence, the arrangement of the power transmission system of the conventional all-terrain vehicle raises a problem that the vehicle is hard to ride.